In recent years, magneto-optical discs boast increasingly high storage density features as computer storage devices and package media for music and image information. Information is written on a magneto-optical disc by either of the following two methods: an optical modulation method of storing information by modulating light intensity of a laser beam according to the information, and a magnetic field modulation method of storing information by modulating an external magnetic field according to the information. The magnetic field modulation method yields a higher storage density than the optical modulation method, and is adopted in many of the commercialised magneto-optical storage and reproduction devices.
According to the magnetic field modulation method, a high frequency magnetic field needs be applied to a storage film of the medium, and a magnetic head therefore needs be disposed in a proximity of the storage film. For these reasons, as shown in FIG. 25, a conventional storage medium includes a translucent dielectric layer 62, a storage layer 63, a non-magnetic layer 64, a reflection layer 65, and a protection layer 66 provided in this order on a translucent substrate 61. A magneto-optical storage and reproduction device includes an optical head (not shown) and a magnetic head 70. The optical head has an objective lens 71, and structured to emit a light beam 72 onto the magneto-optical disc from the side on which the translucent substrate 61 is provided. The magnetic head 70 is constituted by a magnetic core 701 and a coil 702. Since the translucent substrate 61 is as thick as about 1 mm, the magnetic head 70 is provided on the opposite side to the optical head with respect to the translucent substrate 61 so as to be positioned near the storage layer 63.
If the magnetic head is provided on the opposite side to the optical head with respect to the storage medium as noted above, problems occur that the device increases in size and that a pasted disc cannot be used.
Also, if the magnetic head is provided on the same side as the optical head with respect to the storage medium, problems occur that the magnetic field produced by the magnetic head at the position of the storage layer becomes smaller and that information cannot be stored.
By contrast, Japanese Laid-Open Patent Application No. 63-204533/1988 (Tokukaisho 63-204533) discloses an arrangement in which a optical head and a magnetic head coil are provided on the same side with respect to the storage medium, and a ferromagnetic material is provided either on the opposite side to the optical head and the magnetic head coil with respect to the storage medium or in the storage medium.
That conventional arrangement has an object to prevent an external magnetic field flux from diverging when information is being stored on the storage medium. However, a large electric power is essential to store information on the storage medium, and no commercialised usages has been established so far.
Another conventional approach is to use no external magnetic field in a storing operation. For example, Japanese Laid-Open Patent Application No. 5-182278/1993 (Tokukaihei 5-182278) discloses a direct overwrite method by use of a storage layer and a supplementary layer, and a medium to work the method.
However, in the case of the storing on a magneto-optical storage medium, storage density can be more readily improved with magnetic field modulation than with optical modulation, and there is a popular demand for memory with a larger capacity. Accordingly, a storage medium, device, and method for magnetic field modulation which can use both sides of the disc need be developed.